There are indeed some differences you can identify in terms of the level of impairment you see in habitual and novice users. It really depends on the individual. The one thing we've learned in doing studies is that it is very difficult to set a threshold at which impairment occurs in most people and does not occur in other people.
In fact, if you look at the data, you'll see that the best point of discrimination is zero. People who have no cannabis in their system perform better than people who have levels of even one nanogram in their system. Also, you cannot discriminate in terms of people who have different levels of cannabis in their systems. That's been done a couple of times now with real people who have consumed cannabis ad lib, on their own, that is, in the real world. Two nanograms can be a level of impairment in a lot of people. There's no question about that.